Why doesnt it work? Multiple MCs via AS

Im trying to create a MC where i have multiple instances of another MC in a pattern.
I tried this code:

    _root.createEmptyMovieClip("vzorec_vrat", this.getNextHighestDepth());
        var k:Object;
        for (i = 0; i<stKasetVer; i++) {
            for (j = 0; j<stKaset; j++) {
                k = _root.vzorec_vrat.attachMovie("kaseta", "kaseta"+i+""+j, this.getNextHighestDepth());
                k._x = i*520+presledekHor;
                k._y = j*400+presledekVer;

        _root.vzorec_vrat._x = zac_X;
        _root.vzorec_vrat._y = 480-visinaPr;

but nothing shows up in the movie. If i simply call

       _root.vzorec_vrat.attachMovie("kaseta", "kaseta_mc", this.getNextHighestDepth());

the instance of that MC shows up fine so something’s wrong with the way im showing mupltiple instances. But what?